The present invention relates to generating steam for heating and/or frothing liquids, especially milk.
EP 0 781 520 Al discloses a steam generating device of this type. In the Cappuccino position, that is the position where steam for frothing milk is desired, switches S1 and S2 adopt the position in which the tube heating element of the continuous flow heater is energized. Simultaneously, the electric control means is actuated by way of the input so that the electric motor drives the piston pump and pumps water into the water pipe of the continuous flow heater, where it evaporates and is conducted via the output line to the steam nozzle. The steam is accelerated in the steam nozzle and will then exit at the end of the steam nozzle (FIGS. 1 and 2 of EP 0 781 520 Al).
Thus, the instant when the pump starts to deliver water is the very instant when the heating device begins to heat the water heater. The result is that the water heater is not yet so heated up as to be in a position to entirely evaporate the volume of water it receives. The final result is that hot water or a hot steam/water mixture exits from the steam nozzle. This impairs the quality of the frothing result.
Further, it is known from Espresso machines that a waiting time is necessary after switch-on of the Espresso machine until the continuous flow heater has reached its operating temperature that is required for the generation of steam so that only steam will exit from the nozzle. This necessitates a signalling means which indicates when the frothing operation can be started. This arrangement causes prolonged waiting times and disadvantages of use of the appliance.
An object of the present invention is to provide a steam generating device for heating and/or frothing liquids, especially milk, for a hot beverage machine which produces steam by especially simple means in a shortest possible time. An additional objective is to provide a method therefor which permits automatically controlling the steam generation after switch-on of the appliance with equally simple means.
The provision of an intermediate tank with an overflow chamber (which represents in the system a sort of buffer for the intermediate storage of water or a delay time element) between the pump and the steam pipe of the continuous flow heater permits achieving by especially simple means that water will not exit from the discharge of the intermediate tank and propagate to the continuous flow heater until the continuous flow heater has reached its operating temperature. The rate of delivery of the pump, the filament power of the continuous flow heater and the accumulation volume of the intermediate tank must be so conformed to one another that water will only be conducted into the continuous flow heater when the latter has reached the operating temperature that is required for the generation of steam. Thus, the present invention eliminates the need for sophisticated time delay elements because, according to the present invention, activation of the steam generating device causes simultaneous switch-on of the pump and the heating device of the continuous flow heater, and the intermediate tank will retain the water delivered until the continuous flow heater has reached its operating temperature necessary for steam generation.
Instead of an intermediate tank, it is also possible to choose a hose portion of a certain length in which the quantities of water delivered by the pump are received until the continuous flow heater has reached its operating temperature. In this event, too, it would be advantageous that the hose-side end to the continuous flow heater has an elevated position vis-à-vis the intake to prevent a too quick propagation of water into the continuous flow heater. The hose acts as a type of overflow chamber also in this arrangement.
The overflow chamber in one embodiment may extend substantially horizontally in its longitudinal direction and may be separated by a vertical wall from the discharge so that the excess water flows over the wall and into the discharge of the intermediate tank and, thus, into the steam pipe only when the overflow chamber is filled with water. In this embodiment, the intake and the discharge may be designed at the same level or at different levels. If, however, according to another embodiment, the intermediate tank extends vertically in its longitudinal direction, the discharge is required to be arranged at the chamber at the level which defines the chamber volume that is sufficient to take up so much water that the heating device has reached its operating temperature exactly when overflow begins.
It is possible that the intake is arranged above the discharge on the intermediate tank, that means, the actual intermediate tank is positioned below the intake and also below the discharge. In another variation, wherein the intake is designed on the bottom of the intermediate tank, the actual intermediate tank is arranged above the intake. In this event, the discharge is also disposed in the longitudinal direction of the intermediate tank above the intake.
The intermediate tank may include a discharge which has a pressure relief valve that opens in the presence of inadmissible excess pressure in the intermediate tank and discharges steam or water to the atmosphere. Inadmissibly high pressures in the intermediate tank which might cause explosion or leakage of the intermediate tank or of parts of the entire water/steam system are this way prevented.
In some embodiments, the intermediate tank is succeeded by a pressure chamber in which a piston that is displaceable in opposition to the force of a spring is guided, the said piston shutting off the discharge to the atmosphere in the closed position and opening the pressure chamber with the discharge in the open position. A pressure relief valve of this type is especially easy to manufacture and can be integrated especially simply into the pressure chamber that is directly adjacent to the intermediate tank. Advantageously, the pressure chamber and the intermediate tank form a chamber which is configured as a cylindrical bore. This chamber may be provided especially easily.
The piston may have an indentation in the shape of a hollow chamber on its side facing the intermediate tank. This indentation serves as a pressure cushion and, additionally, as a steam accumulator. The purpose of the pressure cushion containing a steam/air mixture is to prevent the steam which flows back into the intermediate tank when the pump and the continuous flow heater are switched off from immediately condensing in the cold water.
Another aspect of the invention features a method for steam generation in a steam generating device wherein both the pump and the continuous flow heater switch on after the appliance is switched on. The water delivered by the pump is now initially supplied to the intermediate tank configured as overflow tank and accumulated therein until the continuous flow heater has reached its operating temperature required for the steam generation. The rate of delivery of the pump and the accumulation volume of the intermediate tank are conformed to the filament power of the continuous flow heater so that water will not flow into the steam pipe until the continuous flow heater or the steam pipe has reached its temperature that is required for the generation of steam. Exclusively steam which exits at the steam tapping point, preferably from the steam nozzle, is generated by the method of the present invention in a time-delayed manner after the appliance is switched on. The method of the present invention does not require sophisticated electronic time elements or other electronic sensing means which indicate to a user at what point of time the continuous flow heater has reached its operating temperature so that the appliance activates the pump only then, or manual activation is rendered possible.
In some cases the method permits evacuation of the intermediate tank after each process of steam generation in order to make the intermediate tank available again in its function for the next case of use.
One embodiment is illustrated in the accompanying drawings and will be explained in detail hereinbelow.